A person breathes into and from a spirometer (volume 12 liters) containing 10% helium gas mixture. After equilibration, helium concentration of expired gas was found to be 6.67%. His ERV is 4.2 liters. What is his residual volume? (Hint: V1C1=V2C2)

Correct Answer: C

Rationale: Residual volume (RV) is the air left after maximal expiration, not measurable by spirometry but calculable via helium dilution. Here, a 12 L spirometer with 10% helium (C1 = 0.1) equilibrates with lung volume (initially FRC). Post-equilibration, expired gas is 6.67% helium (C2 = 0.0667). Using V1C1 = V2C2 (helium conservation), V1 = 12 L, C1 = 0.1, C2 = 0.0667: 12 × 0.1 = V2 × 0.0667, so 1.2 = V2 × 0.0667, V2 = 1.2 / 0.0667 ≈ 18 L. V2 is total gas volume (spirometer + FRC). FRC = V2 - V1 = 18 - 12 = 6 L. FRC = ERV + RV, and ERV = 4.2 L, so RV = FRC - ERV = 6 - 4.2 = 1.8 L = 1800 ml. This assumes equilibration at FRC (post-normal expiration), common in such problems. The 1800 ml matches helium dilution principles, where dilution reflects unexpired lung volume, confirming RV amidst the options.

Correct Answer: B

Rationale: Idiopathic pulmonary fibrosis (IPF) scars the lung interstitium, reducing elasticity and volumes. Total lung capacity (TLC) decreases (e.g., from 6 L to 4 L) as stiff lungs resist expansion. FEV1 and FVC both drop due to restricted capacity, though their ratio (FEV1/FVC) stays normal or high (≥80%). Arterial PO2 (PaO2) falls (e.g., from 75-100 mmHg to 60 mmHg) due to impaired diffusion across thickened alveoli, causing hypoxemia. However, total pulmonary vascular resistance (PVR) increases as fibrosis compresses and obliterates capillaries, narrowing the vascular bed and raising resistance to blood flow. This can strain the right heart, potentially leading to cor pulmonale, a known IPF complication. Among these, only PVR exceeds normal levels, reflecting the disease's vascular impact, while volumes and oxygenation decline, aligning with IPF's restrictive pattern and distinguishing it from healthy physiology.

Correct Answer: D

Rationale: Per Fick's law (Rate = A × D × ΔP / d), diffusion decreases if surface area (A) drops (e.g., emphysema destroys alveoli, halving A halves rate), diffusion distance (d) increases (e.g., pulmonary edema doubles d from 0.5 to 1 μm, halving rate), or partial pressure gradient (ΔP) falls (e.g., hypoventilation lowers alveolar PO2). Decreased pressure coefficient' likely means ΔP; reducing it (e.g., from 60 to 30 mmHg) slows diffusion. Increased lung fluid thickens the barrier, adding resistance beyond distance (e.g., protein debris). All factors reduced A, increased d, lowered ΔP independently and collectively cut diffusion, as seen in hypoxemia from edema or fibrosis. Diffusion coefficient (D) is unchanged here. Each aligns with clinical scenarios impairing O2 transfer, making all the above' correct, reflecting multiple pathways to reduced gas exchange efficiency.

Correct Answer: B

Rationale: The PAO2-PaO2 gradient (alveolar-arterial O2 difference) is normally ~5-10 mmHg due to efficient diffusion. In pulmonary fibrosis, thickened alveolar walls impair O2 transfer, dropping PaO2 (e.g., to 60 mmHg) while PAO2 (~100 mmHg, per alveolar gas equation) stays closer to normal, widening the gradient (e.g., 40 mmHg). During exercise, a normal person's ventilation and perfusion match, keeping the gradient small despite higher O2 use. Anemia lowers O2-carrying capacity, not diffusion, so PaO2 ≈ PAO2, maintaining a normal gradient. At 5000 m, low atmospheric PO2 reduces both PAO2 and PaO2 (e.g., 50 vs. 45 mmHg), keeping the gradient small. Fibrosis's diffusion barrier creates the largest gradient, as O2 struggles to cross, a hallmark of restrictive disease affecting gas exchange, unlike other scenarios.

Correct Answer: D

Rationale: Pulmonary fibrosis, a restrictive disease, stiffens lungs via interstitial scarring. FEV1/FVC is normal or high (≥80%) as FEV1 and FVC drop proportionally true. Vascular resistance rises as fibrosis compresses capillaries true. Peak expiratory flow (PEF), corrected for reduced volume, can be normal or high, as airflow isn't obstructed true. Residual volume (RV) decreases (e.g., from 1.5 L to <1 L) in fibrosis due to stiff lungs limiting all volumes, not increases as in obstructive diseases (e.g., COPD) false. Increased RV contradicts restrictive physiology, where elasticity loss shrinks residual air, making it the inconsistent value, while others align with fibrosis's impact on mechanics and circulation.